Mutations in the PKD1 gene are responsible for 85% of ADPKD. Its full length, encoded protein, polycystin-1, a 4303 aa membrane protein normally expressed at high levels at the basal membrane focal adhesions of the ureteric bud epithelium of developing human kidneys and at lower levels in the apical-lateral cell-cell adherens junctions of normal adult collecting tubule epithelia. The large extracellular N-terminal domain interacts with environment and elicits multiprotein complex formation and signal transduction via its shorter (equal to or >200aa) intra-cellular C-terminal domain (CTD). Several lines of evidence suggest that phosphorylation of the polycystin-1 CTD is central to regulation of its function and specific c-src (Y4127), FAK (Y4127), PKA (S4252) and PRKX (S4166) sites have already been mapped. Our studies are designed to test the hypothesis that polycystin-1 phosphorylation is critical for normal function and that truncating mutations in ADPKD result in aberrant signal transduction: 1. We will determine under which conditions polycystin-1 becomes phosphorylated by examining normal and mutant cells as well as transfected cells under different conditions of matrix adhesion, growth factor-stimulated proliferation and multiprotein complex formation. 2. We will analyze the downstream and functional effects of polycystin-1 phosphorylation, the consequences of mutations on downstream functions and will test putative target modulators for correction of function. 3. We will analyze the importance of SH3 and WW sites in the polycystin-1-CTD and the role of phosphorylation of the flanking sequences with regard to normal and mutant polycystin- 1 function. Completion of these studies will significantly advance our understanding of the phosphorylation-mediated function of polycystin- 1, which pathways are critical in generating the ADPKD phenotype and thus ultimately lead to rational, molecular drug design for new therapeutic approaches aimed at retarding cyst expansion in polycystic kidney diseases.